FIG. 1 shows a pair of semiconductor chips 101, 102 coupled together by a serial link 110 having a data signal line 103 and a clock signal line 104. The transmitting unit 101 sends a tx data signal 105 to the receiving unit 102 along tx data signal line 103. The receiving unit 102 uses a tx clock signal 106 that is sent along tx clock signal line 104 to receive the tx data 105.
That is, in the example of FIG. 1, the receiving unit 102 clocks the tx data signal 105 on the rising edge of the tx clock signal 106. The tx clock signal 106 may be referred to as a quadrature clock because the phase of its rising edges are 90 degrees away from the rising edges of the tx data signal 105 (using the tx data signal 105 as a phase reference).
A problem with serial links, particularly as their frequency of operation rises, is the presence of skew 109 between a tx data signal 107 and a tx clock signal 108 when it is received at the receiving unit. Skew 109 is any phase relationship between the edges of the tx data signal 107 and tx clock signal 108 other than the nominal or “designed for” phase relationship (such as 90 degrees, using the tx data signal 107 as a phase reference).
Skew may arise because the transfer function and/or trace length of the data signal line 103 is different than the transfer function and/or trace length of the clock signal line 104. For example if the data signal line 103 is shorter or has less capacitance than the clock signal line 103, the rising edges of the tx clock signal 108 will have more than 90 degrees of phase shift with respect to the rising edges of the tx data signal 107.
For a given difference in transfer function and/or trace length between the data and clock signal lines 103, 104, greater skew is observed between the tx data signal and tx data signal as the frequency of operation of the serial link 110 increases. That is, the differences between the signal lines 103, 104 have an effect on the delay of the signals as they propagate from the transmitting unit 101 to the receiving unit 102. As the frequency of the serial link's operation rises, the delay represents a greater percentage of the data signal's pulse widths.
As skew 109 increases the performance of the serial link degrades. That is, because the receiving unit 102 uses the tx clock signal to clock the reception of the data carried by the tx data signal 107, the “misposition” of the tx clock signal 108 edges causes the receiving unit 102 to occasionally clock incorrect data.